The following video explains the main features of the Castle of San Felipe before its demolition.
In front of the 4 bastions the 4 counterguards were built and, between them, the revellines were built. More at the forefront, as the first line of defense, the rear windows or the outer redoubts.
On the sea line, 4 forts were built, the one in Malborough, in St. Stephen’s Cove, which prevented a possible positioning of enemy artillery on the Cerro del Turco, and they extended the fort of San Carlos at the entrance of the port. If Fort Felipet and St. Philip’s Castle were overrun, Argyll’s and Anstruther’s forts were the last line of defense.
All these surface defenses were reinforced by more than 8 kilometers of underground galleries that connected the central positions with the outer ones, under cover of enemy fire. Under the ground we find the defense and combat galleries, which together with the communication and branch galleries allowed the defenders to move, thus multiplying their defensive capacity. A whole network of countermines, stoves and cameras prepared to explode as the enemy passed made up the underground defenses of San Felipe Castle.